The 4ESS switches in the IXC network are provided with a Common Network Interface (CNI) ring for providing connectivity to the IXC signaling network. CNI provides both Common Channel Interoffice Signaling No. 6 (CCIS6) and CCS7 capabilities for destination routing of signaling messages between nodes within the signaling network. A 4ESS CNI-based signaling system failure may result in signal point isolation. Such signaling isolation of a 4ESS might occur, for example, if there were a CNI ring failure or a failure of both A-links connecting the 4ESS switch to Signaling Transfer Point (STP) mates.
Until recently, failure of the 4ESS CNI-based signaling system was transparent to AT&T customers in the LEC network for two reasons. First, trunk signaling from the LEC network to the 4ESS within the AT&T Switched Network (ASN) were primarily inband multifrequency (MF). Second, the Alternate Signaling Transport Network (ASTN) backed up signaling traffic among 4ESS switches within the ASN. Thus, calls utilizing inband signaling were delivered to the 4ESS switch in the IXC network independently of the state of the CNI-based signaling system interface with backup provided by ASTN for signaling occurring inside the ASN. A similar scenario is applicable to calls egressing from the ASN to the LEC.
The ASTN is an AT&T CCS7 backup network to protect 4ESS switches in the ASN against isolation of intertoll trunk signaling. ASTN is a logically (different software) and physically (different hardware) separate signaling network from the CCS7 network. In the ASTN, the 4ESS toll switches are partially connected by F-links (without the use of an STP). At the two ends of the F-link, the isolated 4ESS is called the "victim" and the aiding backup 4ESS is called the "helper". The ASTN messages carried on the F-links are supported by the LAPB protocol.
Assuming that a 4ESS victim, V, is CCS7 isolated due to the failure of its A-link set and/or CNI, and CCS7 trunks are being established between this isolated switch and another normal 4ESS switch, S, then backup signaling is performed for messages from V to S and from S to V. For messages from V to S, switch V selects a helper, H, and sends ASTN messages to switch H (destined to S) via the F-links. Switch H places the received ASTN messages into a CCS7 envelope and sends them to the destination switch S via the CCS7 signaling network. For messages from S to V, switch S selects one of the victim's helpers H, and sends an ASTN message enveloped with a CCS7 header to H via the CCS7 signaling network. Switch H unwraps the CCS7 header and sends the embedded ASTN message to V via the F-links.
As of the early 1990's, both the IXCs and LECs maintained their own common channel signaling networks. However, at this time, common channel signaling messages were not typically capable of being passed between the IXC and LEC networks. Thus, trunk signaling from LEC switches to the 4ESSs in an IXC network, such as the ASN, was primarily inband MF signaling.
With the advent of out-of-band CCS7 signaling for control of circuit-switched connections between the LECs and the IXCs, there arose the risk that a CNI-based signaling system failure would block calls attempting to enter or leave the ASN. A CNI failure at a 4ESS receiving voice traffic from an LEC End Office (EO) or Access Tandem (AT) will prevent that 4ESS from exchanging the necessary call signaling information with the EO or AT. Moreover, the above-described ASTN cannot backup internetwork signaling in the event of such a failure.
In the 1990-91 time frame, a standard CCS7 interface at the 4ESS switch was studied and proposed for backup signaling when the A-links and/or CNI of the 4ESS fail. This standard interface was known as the Alternate Signaling Interface (ASI).
At the 4ESS switch, the ASI was proposed as a separate CCS7 signaling unit in addition to the normal CNI interface. Standard E-links would be used to connect the ASI of a 4ESS switch to the STP in a remote region. When a 4ESS switch cannot send/receive CCS7 messages via the A-links due to the failure of A-links and/or CNI, the ASI would be turned on, and the E-links carry CCS7 (backup) messages between the 4ESS and the remote STP. The ASI proposal, however, was rejected due to implementation complexity and the high capital costs of E-links at the 4ESS switches.
There is thus the need to protect Network Interconnect (NI) traffic such as SS7 Integrated Services Digital Network User Part (ISUP) traffic in a cost-effective manner. ISUP traffic is used, for example, for call setup and release of CCS7 controlled voice trunks. It has been estimated that the ASN may experience about 20 4ESS isolations (over 5 minutes) per year. This could place millions of calls per year at risk.